Temperatures at these high latitudes already are
climbing “at about twice the global average,”
notes F. Stuart Chapin of the University of
Alaska in Fairbanks.

The newest data on the advance of northern, or
boreal, forests come from the eastern slopes of
Siberia’s Ural Mountains. Here, north of the
Arctic Circle, relatively flat mats of
compressed, frozen plant matter – tundra – are
the norm. This ecosystem hosts a cover of
reflective snow most of the year, a feature that
helps maintain the region’s chilly temperatures.
Throughout the past century, however, the leading
edges of conifer forests have creeped some 20 to
60 meters up the mountains and begun overrunning
tundra, scientists report in an upcoming Global
Change Biology, now available online.

Conifers here now reside where no living tree has
grown in some 1,000 years, points out ecologist
Frank Hagedorn of the Swiss Federal Institute for
Forest, Snow and Landscape Research in
Birmensdorf.

Ecologists and climatologists are concerned
because the emerging forest data suggest that the
albedo, or reflectivity, of large regions across
the Arctic could change. Most sunlight hitting
snow and ice bounces back into space. But convert
a white landscape to open sea water or boreal
forest, and the surface suddenly becomes a great
collector of solar energy.

Sea-surface ice already is melting in the Arctic
and polar ice sheets are thinning. Warming
threatens to further degrade these solar
reflectors. So does the advance of boreal
forests, Chapin says.

“The effects of vegetative changes will be felt
first and most strongly locally – in the Arctic,”
he says. If the albedo there drops broadly, this
could further aggravate warming there and
underway elsewhere across the planet.

Tree rings from the Arctic Urals show that since
the 15th century, many of the primary tree
species – Siberian larch (Larix sibirica Ledeb.)
– have grown in a stunted, shrubby form, sporting
multiple spindly trunks. This adaptation to harsh
conditions helps the trees weather wind and snow.
But the trees invest so many calories into making
multi-stem clusters, Hagedorn says, that they end
up puny and unable to make seeds. The inability
to reproduce has inhibited the stand’s spread.

After about 1900, the local Siberian larch began
to switch from their creeping, multi-stem form to
tall trees with a more upright posture, though
sometimes with up to 20 stems, Hagedorn and teams
of Russian and Swiss collaborators found. Over
time, new trees emerged with a single, upright
trunk, at the same time bulking up with more
biomass than shrubby, same-age kin. Overall, 70
percent of upright larches are no more than 80
years old. Since 1950, 90 percent of local
upright larches have been single-stemmed. This
forest’s movement into former tundra coincided
with a nearly 1 degree Celsius increase in summer
temperature and a doubling of winter
precipitation.

“That’s a good cocktail for growth,” says arctic
plant ecologist Serge Payette of Laval University
in Quebec. Whether a tree grows up versus out
depends on survival of its uppermost, or apical,
buds. Good snow cover will protect those buds
from winter damage, he says. Only if they are
destroyed will the surviving lateral buds push
growth horizontally, he explains.

Spruce are North America’s more common boreal
species at polar tree lines, Payette says. Some
of these also assume a shrubby form, creating
what he calls “pygmy forests” perhaps a meter
high. But he has witnessed some of these trees
assuming new, upright postures as areas warm and
get wetter.

This process can create the “mirage” of tree line
advance, he says. In fact, the trees may not move
at all; in-place populations may simply recover
from chronic stress and resume growth until they
reach their normal height and mass.

Ecologist Andrea Lloyd of Middlebury College in
Vermont has been studying the health of boreal
tree lines throughout the warming Arctic. As in
the Urals, warmth seemed to spur American larches
to grow faster. “I’ve also seen spruce advancing
upwards,” she says, climbing up mountains to form
dense stands.

But that’s only part of the story, she finds.
Even where stands are advancing, “if you look at
individual trees, some are starting to decline.”
They’re growing increasingly slowly. Sometimes,
as growth slows, tree numbers within a stand may
be increasing. “It’s a paradox,” she acknowledges.

Forest ecologist Glenn Juday of Alaska-Fairbanks
and his student Martin Wilmking have recorded
similarly perplexing data from tree rings in
2,600 trees along two mountain ranges in polar
Alaska. As the environment warmed, 42 percent of
the trees grew more slowly and 38 grew more
quickly.

Too little water seems a bigger factor affecting
tree growth than temperature, although warming
can foster drought, Juday reports. Indeed, as the
Arctic warms, it will likely become drier, he
says. “So we can expect that at least in the
western North American Arctic, there are going to
be sites that eventually will get too dry to grow
trees.”

But their loss isn’t likely to compensate for the
tundra lost to trees, at least in Arctic-warming
potential. Indeed, the loss could further perturb
the global climate because boreal forests
currently store huge amounts of carbon once
emitted as carbon dioxide, a greenhouse gas. As
the trees die, their carbon could be released
into the air. Meanwhile, until they fall over and
decompose, they’ll continue to serve as
low-albedo solar collectors.

The threat of tundra displacement by trees has
largely escaped notice, Juday says. And indeed,
boreal forest advances in Alaska have been
modest, at best. One reason: Seeds don’t normally
travel far in the Arctic, and even when they land
on tundra, the dense mats normally resist
implantation.

However, a dry summer and warm September last
year allowed a fire to ignite 100,000 hectares
(about 250,000 acres) of Alaskan tundra. The huge
footprint of disturbed land is now ripe for seed
implantation. Fortunately, Juday says,
seed-bearing boreal forests are on the other side
of a mountain range from the scarred landscape.

Warming has changed the climate of a huge and
growing span of tundra so it now hosts a
temperature and moisture level that would support
forests, if the seeds ever arrived, Juday notes.
“Today, if you planted a tree – in some cases
very far up from the current tree line – it would
survive in many parts of the tundra.” Just 40
years ago, he says, it wouldn’t.